Regenerative capacity of the heart is mediated through multiple distinct populations of stem cell types that are the subject of ongoing intense study. In the past decade, isolation and characterization of cardiac progenitor cells (CPCs), mesenchymal stem cells (MSCs) and endothelial progenitor cells (EPCs) has provided substantial insight to the capabilities of stem cells to rebuild the damaged heart and advance clinical therapy. Clinical trials have proven the efficacy and safety of autologous and allogeneic CPC and MSC delivery to human patients, yet improvements in cardiac function and reduction in scar tissue remain modest and far below that needed for restoration of normal functional output. This proposal overcomes these current cell- based limitations with two novel methods for improving myocardial repair: 1) CardioClusters, a three- dimensional microenvironment consisting of CPCs, MSCs and EPCs, and 2) CardioChimeras, the product of cellular fusion between two stem cell populations. The innovation of this proposal is the creation of CardioClusters and CardioChimeras with the ability to capitalize upon beneficial attributes of multiple human stem cells from a single patient providing a clinically relevant translational strategy. The short-term goal of this proposal will determine the enhanced proliferation, growth, survival and commitment potential of CardioChimeras and stem cells in CardioClusters, which are optimized for improving cell-based therapy. Accomplishing the stated aims of this proposal will yield the construction and comprehensive characterization of CardioClusters and CardioChimeras. Specific Aims are: 1) CardioClusters exhibit enhanced proliferation, survival and cardiac commitment relative to cardiospheres, single or combinatorial cell populations, 2) CardioChimeras display improved characteristics of growth, survival, secretion of paracrine factors and cardiac commitment relative to non-fused cell populations and 3) CardioClusters as well as CardioChimeras restore myocardial structure and function after intramyocardial injection better than cardiospheres or single / multiple cell suspensions. The significance of these studies is to create novel cell-based strategies engineered to improve current cellular therapy to mitigate ischemic disease. Collectively, studies in this proposal will pave the way for interventional approaches to selectively adapt stem cell behavior and merge beneficial attributes of stem cell populations found within the human heart for prevention of heart failure after cardiomyopathic injury.